swd: remove unused API frequency()

swd: remove unused API frequency()

The specific SWD API to change/query the adapter speed is never
called because the equivalent JTAG API is used in place of it.
In the restructure of struct jtag_interface, the JTAG API is
promoted as global adapter API, thus a specific SWD one is not
anymore required.

Change-Id: I1e810d255b4dfcd5791b4fac8ae1260c31a057fd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4891
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>